Fms-like tyrosine kinase 3 (FLT3), which is also known as FLK-2 (fetal liver kinase 2) and STK-1 (stem cell kinase 1), plays an important role in the proliferation and differentiation of hematopoietic stem cells. FLT3 receptor kinase is expressed in normal hematopoietic cells, placenta, gonads, and brain. However, this enzyme is expressed at very high levels on the cells of more than 80% of myelogenous patients and of a fraction of acute lymphoblastic leukemia cells. This enzyme can also be found on cells from patients with chronic myelogenous leukemia in lymphoid blast crisis.
It has been reported that FLT3 kinase is mutated in 30% of acute myeloid leukemia (AML) and in a subset of acute lymphoblastic leukemia (ALL) as well (Gilliland et al., Blood 2002, 100, 1532-1542; Stirewalt et al., Nat. Rev. Cancer 2003, 3, 650-665). The most common activating mutations in FLT3 are internal tandem duplications within the juxtamembrane region. The point mutations, insertions, or deletions in the kinase domain are less common. Some of these mutant FLT3 kinases are constitutively active. FLT3 mutations have been associated with a poor prognosis (Malempati et al., Blood 2004, 104, 11).
More than a dozen known FLT3 inhibitors are being developed and some have shown promising clinical effects against AML (Levis et al. Int. J. Hematol. 2005, 82, 100-107). It has been reported that some of small-molecule FLT3 inhibitors are effective in inducing apoptosis in cell lines with FLT3-activating mutations and prolonging survival of mice that express mutant FLT3 in their bone marrow cells (Levis et al., Blood 2002, 99, 3885-3891; Kelly et al., Cancer Cell 2002, 1, 421-432; Weisberg et al., Cancer Cell 2002, 1, 433-443; Yee et al., Blood 2002, 100, 2941-2949).
Despite the success in identification of small molecules that inhibit protein tyrosine kinases, there continues to be a need for a safe and effective method of using or administering such compounds, particularly to humans having AML and ALL, including compounds useful for the treatment of FLT-3 mediated diseases.